1. Field of the Invention
The present invention relates to a method and a circuit for an enhanced, particularly error tolerant, control of a multiphase (typically 3-phase) brushless DC motor (brushless direct current motor—BLDC motor) based on permanently measured motion information by evaluating the points in time of the reversal of the induced voltage (zero crossing) within two non-current-carrying motor coils, which in the temporal progression passed the commutation point in an immediately consecutive manner.
2. Discussion of Background Information
BLDC motors are driven with an electromagnetic rotating field, which rotates synchronously with the rotor field. The synchronism of the fields, as well as their phase difference, are critical for the efficiency of the operation of this type of motor. It may, therefore, be appreciated that during operation information on the momentary phase difference of both fields is required at any time.
In BLDC motors having hall elements, these elements generate the required position information. This information is then directly translated into the switching of the motor phases (the commutation), thereby fixedly adjusting the angle between the stator and the rotor field. This corresponds to a singular working point that depends on the type of motor, its speed, and load; an optimal motor operation is guaranteed at this working point only.
In WO-A 01/37419, a method is described for shifting the commutation point of a BLDC motor whose stator coils are fed by a multiphase frequency converter circuit. The frequency converter circuit comprises a power stage, a commutation logic, a phase selector and a phase discriminator. A commutation detector receives on a first input the momentary value of a voltage induced during a non-current-carrying phase, which is detected by a phase selector, and also receives a reference voltage on a second input for comparing both input voltages. The reference voltage may be modified by a commutation shifter in accordance with a predefined function. The commutation shifter receives from a controlled-variable calculator a target value that depends on the target speed of the motor. Due to the commutation shift that depends on the target value, even with high speeds and a heavy load, a high motor torque is available and the variation in torque is kept at a low level.
In patent document EP-B 231 046, a commutation circuit is described, which operates without any sensors, wherein each phase may be switched, depending on voltages which are induced in the coils by the permanent magnetic field of the rotor, by electronic switching elements depending on the state of commutation for performing commutation steps at the negative and/or positive terminal of a DC current source, wherein a comparison signal is generated in the control circuit, which indicates whether the sign of that coil voltage that is not connected to the DC source by means of the electronic switching elements, coincides with a sign that is predefined on the basis of the currently valid commutation state and, in case of non-coinciding signs, the phase is otherwise commutated.
In Japanese patent document, Publication 2002/034277 A, Application No. 2000212209, the error in the motor voltage is maintained at a low level for a change of the motor speed within a single rotation, in that mean values of all measured rotor sector positions of a full rotation of the rotor to determine the time interval from one rotor position to the next by calculation, and errors of the corresponding determined positions are used, on the basis of which the points in time for the supply of energy are controlled.
These methods are less efficient compared to the solution of the present invention, that is, the time between the detection of the change of rotor speed and the activity for correcting the error is longer.